Mutant

Visit Website

Introduction

Mutant is an automated code review and mutation testing tool for Ruby. It makes small semantic changes to your code, such as removing statements, replacing expressions, narrowing method calls or changing literals, and runs the tests that cover each subject. If tests still pass, the alive mutation shows code whose behaviour is not specified by your tests, or code that can be simplified.

Mutant integrates with RSpec and Minitest, selects tests by matching subjects to examples, and runs mutations in parallel. It supports incremental runs against a git revision (--since), which keeps CI fast by mutating only changed code, and its configuration lives in .mutant.yml or command-line options. Licensing: according to the README, Mutant is free for open-source use (with --usage opensource for public repositories), while commercial use requires a subscription of $30 per month or $250 per year per developer. Version 0.16.2 was released on 21 April 2026, with commits in November 2026.

Features

1. Semantic mutations: Ruby-aware mutation operators.
2. RSpec and Minitest: Integrations for both frameworks.
3. Incremental runs: --since mutates only changed code.
4. Parallel execution: Uses multiple cores.

Pros

1. Free for open source: No cost for public repositories.
2. Rigorous: Ruby-aware semantic mutations.

Cons

1. Commercial licence: Paid subscription for commercial use.
2. Strictness: Requires disciplined test selection and can be demanding on legacy code.

Usage and Links (Example)

gem install mutant-rspec
bundle exec mutant run --usage opensource --include lib --require my_gem --integration rspec -- 'MyGem::Pricing*'
bundle exec mutant run --since main --usage commercial -- 'MyApp::Billing*'

Resources:

• Mutant website

Pricing

Free for open source (use --usage opensource for public repositories). Commercial use requires a subscription: $30/month or $250/year per developer, per the project README.

Recommended for?

1. Ruby teams wanting strict mutation testing in CI.
2. Open-source Ruby gems, which can use Mutant for free.

Conclusion

Mutant is the most rigorous mutation testing tool for Ruby; it is free for open source, while commercial teams need a subscription.